92-95 Civic owners -a Q
 

I notice that my gas guage barely moves untill I hit around 100 miles on the odometer. It drops almost a good 1/4 inch or so the next time that the car is started, but not really all that drastically when I drive a constant trip of over 100 miles. It only seems to just plummet when I take short trips and restart the car every 20 or so miles. ( And I know that starting the car does not burn up that much fuel )
What is causing this ?
Is the float getting hung up on something ?

my civic would do that. its like the gas gauge is maxed out and it doesnt read almost 3 gallons past the full mark.

Same here. Not a very precise unit.

I forgot the exact name for the type of circuit that honda uses for its fuel gauges....it updates slowly and isn't linear though. It's basically a reference.

same here. it's not for accurate readings, just general status of your fuel amount.

the reason for the sharp drop after 100 miles when the engine is cold is that your gas is cold as well, if you drive 20 or so miles in a single shot, almost all of your fuel will have passed thru the fuel rail that is bolted to the head of your engine, where it has had a chance to warm up, and then is sent back to your fuel tank, warm fuel expands.

^ I doubt that's it.

Honda uses a simple gauge and wire coil variable resistor, operated by a float. The gauge moves so slowly because it has a fairly weak coil to move the needle, and a volume of viscous fluid tied to the needle shaft to dampen movement. Mild pressure against a thick fluid results in slow progress. If they didn't dampen the reading somehow, the gauge needle would bounce around whenever the fuel sloshed around in the tank.

The gauge is non linear - same phenomenon in 88-91's.

I can typically tell if it's going to be a good gas tank if the needle does not move off the F mark until after at least 100 miles. The first time the needle touches the 1/2 mark is typically a true indicator of half my gas being gone. I watch the gauge like a hawk, and try to make my total tank distance at least twice what the reading is the first time the needle touches the 1/2 mark. Most of the time this works.

look at how much fuel expands when heated even just a few degrees, then get under your car and feel the gas tank, and how cold it is, drive 30 miles or more and feel the gas tank again, it will be warmer, I've done it.

I have a 94 accord, and the fuel gauge works the same... dont move much at all for first 100 miles...

So you're suggesting... What? That the density of the gasoline changes enough to make the fuel sender float neutrally boyant? Or just that it changes the level in the tank?
I think your solution is way more complex than it needs to be. The variable resistor used for fuel level senders is far from a precision instrument. Combined with the odd body contours that fuel tanks generally follow, you get strange patterns of needle movement relative to actual fuel volume.

yea but its not 3 gallons worth. realistically how much fuel will ever be in the fuel lines?? how much fuel will ever get above 100degrees?

I don't know for sure about 92-95, but for 89-91 Honda's, they have a couple of characteristics which are common. I have a close association with 4 different cars and they all are pretty similar when it comes to the gauges. The first 50 miles is free, the gas gauge doesn't even see the change. The second 50 is generally good for about 1/8 of a tank. The second 100 generally drops like a rock, to about 1/2 a tank. The last 1/2 of the tank holds for about 50 and then it drops quickly to about a 1/4 tank.

All 4 cars are Honda Wagon's and the tanks hold about 12 1/2 gallons of gas, from stone empty. The gauges don't seem to see the last 2 1/2 gallons of gas, but I expect that much is required to submerge the fuel pickup.

The sensor in the tank is not linear because of it's design. I suspect that the readings are considered more of an aproximation, with don't let it run on empty for to long, being the primary consideration.

The shape of the fuel tank is a significant contributor to the non-linear characteristics of the fuel gauge.